4.5 billion years ago, our earth formed. Hydrogen and Helium were the first gases.
During the Jurassic Period, the climate was generally warmer by 5 to 10 degrees Celsius, and atmospheric carbon dioxide levels were likely four times higher.
I experienced the Ice Age. During the ice age, atmospheric carbon dioxide levels were at 180 parts per million, CO2 also became low.
After the Ice Age, plants became firmly established on land, during the Carboniferous Period. Oxygen made up 20 percent of the atmosphere.
The Earth cooled down eventually, and much of the carbon dioxide was replaced by oxygen.
Today, the current composition of the Earth's atmosphere is roughly 80% nitrogen and 20% oxygen.